Reducing reflectivity on a semiconductor wafer by annealing aluminum and titanium

ABSTRACT

The present invention provides methods of producing an anti-reflective layer on a semiconductor wafer/device and wafers/devices including that anti-reflective layer. The anti-reflective layer is produced by annealing layers of titanium and aluminum on a wafer/device to provide a roughened surface that significantly reduces reflectivity to improve the accuracy and definition provided by optical lithography processes.

This is a division of application Ser. No. 08/610,595, filed Mar. 7,1996, now U.S. Pat. No. 5,838,052, which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to methods of reducing reflectance at thesurface of a semiconductor wafer. More particularly, the presentinvention provides a method of producing an anti-reflective layer or asemiconductor by annealing titanium and aluminum.

BACKGROUND OF THE INVENTION

Patterning layers of photoresist using optical lithography to providedesired circuits or other structures on a semiconductor wafer is known.The photoresist material is imaged using a mask having the desiredpattern after which a portion of the resist is removed to expose theunderlying layer of the semiconductor wafer. Additional processing maythen be performed, such as depositing materials in the exposed areas,etc.

To increase speed and performance of integrated circuits, it isdesirable to continually decrease the dimensions of structures, such astraces, etc. that are placed in the exposed areas on the wafer. Onefactor that limits the dimensions in optical lithography is reflectionof the light used to expose the photoresist. The reflected listadversely affects the control over dimensions by, for example, exposingphotoresist outside of the desired areas which can lead to undercutting.Another undesirable effect of reflections from the underlying surface isthe creation of standing waves if the illuminating light ismonochromatic. The standing waves can vary the development of the resistmaterial along the edges of the pattern, thereby decreasing the imageresolution.

The additional exposed photoresist will result in variations in thedesired dimensions of the areas exposed in the photoresist material. Asa result, the areas exposed in the photoresist will also vary. Althoughthose variations can be accounted for in the design of the patterns,they do limit the minimum dimensions that can be accurately patterned.

Reflectance problems are particularly troublesome when the layerunderneath the photoresist is aluminum. Aluminum is widely used in themanufacture of integrated circuits because of its low melting point,high conductivity and low cost. It is, however, highly reflective whichenhances the reflectivity problems discussed above.

One attempt at reducing reflectance of aluminum beneath a layer ofphotoresist to enhance resolution in optical lithography involvesdepositing an anti-reflective coating (ARC) on the aluminum, beneath thephotoresist, to absorb light reaching the aluminum to prevent it fromexposing unwanted areas of photoresist. One example of ananti-reflective coating is amorphous silicon as described in U.S. Pat.No. 5,441,616 to Nanda, et al. Another example is a sputtered layer ofTiN as described in U.S. Pat. No. 5,427,666. When using TiN, however,reflectance is typically reduced to about 20% of the incident light andis only that effective over relatively narrow range of wavelengths.

SUMMARY OF THE INVENTION

The present invention provides methods of producing an anti-reflectivelayer on a semiconductor wafer/device. The anti-reflective layer isproduced by annealing layers of titanium and aluminum on a wafer/deviceto provide a roughened surface that significantly reduces reflectivityto improve the accuracy and definition provided by optical lithographyprocesses.

The annealing can be performed in a nonreactive atmosphere resultingprimarily in the formation of Ti_(x) Al_(1-x). Alternatively, theannealing can be performed in a nitrogen atmosphere where it results inthe formation of primarily Ti_(x) Al_(1-x) and Ti_(x) Al_(1-x) N_(y)where x and y can be the same or different.

One advantage of the annealing processes and products according to thepresent invention is that reflectivity can be significantly lowered overa useful range of wavelengths, preferably from about 100 to about 450nm.

These and other various features and advantages of the invention aremore fully shown and described in the drawings and detailed descriptionof his invention, where like reference numerals are used to representsimilar parts. It is to be understood, however, that the description anddrawings are for the purposes of illustration only and should not beread in a manner that would unduly limit the scope of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one metal stack useful in methodsaccording to the present invention, including an uppermost layer oftitanium deposited on aluminum.

FIG. 2 is a schematic diagram of the metal stack of FIG. 1 afterannealing in an inert gas environment.

FIG. 3 is a schematic diagram of the metal stack of FIG. 1 afterannealing in a nitrogen.

FIG. 4 is a graph of reflectance of an annealed metal stack producedaccording to Example 1.

FIG. 5 is a graph of reflectance of an annealed metal stack producedaccording to Example 2.

FIG. 6 is a graph of reflectance of an annealed metal stack producedaccording to Example 3.

FIG. 7 is a graph of reflectance of an annealed metal stack producedaccording to Example 4.

FIG. 8 is a graph of reflectance of a layer of TiN produced according toComparative Example A.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to methods of providing ananti-reflective coating by annealing layers of titanium and aluminumdeposited on the surface of a semiconductor wafer. The anti-reflectivecoating improves the resolution of images patterned into a photoresistlocated on the semiconductor wafer by reducing the amount of lightreflected off of the surfaces beneath the photoresist.

The anti-reflective layer formed by annealing according to the presentinvention is useful in the production of, for example, semiconductordevices such as memory chips and processors.

The reflectance is reduced due to scattering by the increased surfaceroughness formed upon annealing of the titanium and aluminum layers.Pickering et al, discuss the effects of surface roughness on reflectancein "Characterisation of Rough Silicon Surfaces using SpectroscopicEllipsometry, Reflectance, Scanning Electron Microscopy and ScatteringMeasurements," Materials Science and Engineering, B5, pp.295-299 (1990).In addition to lowering reflectance to a minimum at a given wavelength,the products of the annealed titanium and aluminum layers also reducethe slope of the reflectance curve to provide significantly reducedreflectance over a useful range of wavelengths as discussed in moredetail below.

FIG. 1 depicts one metal stack useful in methods according to thepresent invention. The metal layers are provided on a substrate 10 whichcan be any suitable material, e.g., silicon-based or galliumarsenide-based. The metal stack includes a first layer 12 of titanium, alayer of aluminum 14, and a top layer 16 of titanium. These layers canbe deposited by any means including evaporation, sputtering, CVD,electroplating, etc.

Typically, the thickness of the metal layers 12-16 is based onelectrical considerations, i.e., conductivity, etc. Typical thicknessfor the aluminum layer 14 is about 5000 Å, although the aluminum layer14 can range in thickness from about 2 kÅ to about 12 kÅ depending onthe needs of the circuit designer.

The thickness of the titanium layers 12 and 16 can also vary based onthe need for barrier metallization to promote adhesion, as well as theneed to form the desired anti-reflective coating at the exposed surface.Where the titanium layers are provided primarily to react with thealuminum to produce a desired anti-reflective coating, a preferredthickness for the titanium layers 12 and 16 can range from about 100 Åto about 1 kÅ, more preferably from about 200 to about 750 Å.

It is typically preferred that the layers 12-16 of the metal stack begenerally free of impurities that slow or inhibit the Al--Ti reaction,e.g., silicon.

The upper layer 16 of titanium is essential to produce the desiredsurface roughness required to significantly decrease reflectance and theslope of the reflectance curve. The lower layer 12 of titanium can, insome instances, be eliminated, although results have shown thatproviding a layer 12 of titanium beneath the aluminum layer 14 can havea positive impact in reducing reflectance.

After the layers 12-16 are deposited on the substrate 10 they areannealed under conditions that cause the titanium and aluminum layers toform a layer 18 having a roughened surface comprised at least partiallyof titanium-aluminum compounds on the metal stack as depicted in FIG. 2.To prevent undesired reactions with the exposed titanium layer 16, thechamber containing the semiconductor wafer is preferably flooded with anon-reactive atmosphere. For use in connection with thc presentinvention, "non-reactive atmosphere" is any atmosphere that does notreact with the metal layers under the annealing process conditions. Oneexample of a non-reactive atmosphere useful in connection with theinvention is argon.

In some applications, the entire upper layer 16 of titanium may reactwith aluminum in layer 14, but it is not required that all of thetitanium in layer 16 react to produce the desired decreases inreflectivity. The amount of titanium in layer 16 that does react with thaluminum in layer 14 will depend to some extent on the thickness of thelayers 14 and 16, as well as the conditions under which the annealingtakes place.

The temperature used in the annealing process must be high enough toproduce the required surface roughness in layer 18 required to reducereflectance and the slope of the reflectance curve vs. wavelength todesired levels. Annealing of titanium-aluminum metal stacks onsemiconductor wafers according to the present invention can be performedat temperatures within a range of about 350 to about 550° C., morepreferably about 440 to about 480° C. Duration of the annealing alsoplays a role in formation of the desired anti-reflective coating. It ispreferred that the annealing process occur over about 30 seconds toabout 15 minutes, more preferably from about two to about three minutes.

The temperature and duration of the annealing are based on a number offactors including: a) the thickness of the titanium and aluminum layers;b) formation of a sufficient amount of titanium-aluminum compounds tosignificantly reduce reflected light; c) the need to protect or notdamage structures and materials located on the semiconductor wafer; d)the need to prevent unwanted diffusion between metal layers; and e) theneed to maximize throughput in the process.

In another method according to the present invention, the annealing stepis performed while the chamber containing the semiconductor wafer isflooded with nitrogen. The nitrogen reacts with the exposed surface ofthe titanium layer 116 during the annealing, resulting in the formationof a layer 118 at the surface of the metal stack (including a layer 114of aluminum deposited on a bottom layer 112 of titanium on substrate110) as shown in FIG. 3. By providing nitrogen during the annealing, theformation of Ti_(x) Al_(1-x) N_(y) and Ti_(x) N_(1-x) (where x and y canbe the same or different and, further, where x and y can be 0 to about17, although will predominantly lie within a range from 0 to about 4)are promoted in layer 118. Other compounds may also be formed during theannealing process. The result of this process is a rough surface thatreduces reflectance and the slope of the reflectance versus wavelengthcurve.

Typical wavelengths used to image photoresist materials in opticallithography processes range from about 100 nm to about 450 nm, with theexact wavelength or range of wavelengths being chosen based on a numberof factors including the composition of the photoresist, patterndimensions, resolution, etc. Some wavelengths that are particularlyuseful include 254 nm and 365 nm.

By depositing a layer of titanium over aluminum and annealing thelayers, the present invention provides a method of reducing the amountof light reflected off of a layer of exposed aluminum on a semiconductorwafer from near 100% to at or below about 10% over the range of usefuloptical lithography wavelengths, i.e., 100-450 nm. when measured usingspectral analysis techniques. If the titanium-aluminum layers areannealed in a nitrogen atmosphere, even further reductions inreflectance can be obtained, i.e., down to at or below about 2% over the100-450 nm range.

As an alternative to the processes described above, it is also possibleto perform the annealing in multiple phases, i.e., annealing in an inertgas atmosphere followed by annealing in nitrogen or vice versa.

The following non-limiting examples illustrate some methods of annealingtitanium and aluminum on the surface of a semiconductor wafers.

EXAMPLE 1

In one method according to the present invention, a 500 Å thick layer oftitanium was deposited on a generally planar boronphosphosilicate glasssubstrate using direct current magnetron sputtering. A 5000 Å thicklayer of aluminum was deposited on the titanium layer using directcurrent magnetron sputtering in the same tool, after which a secondlayer of titanium having a thickness of about 500 Å was then depositedover the aluminum layer using direct current magnetron sputtering in thesame chamber used to deposit the first titanium layer. All layers weresubstantially homogenous in composition.

The wafer was then placed in an annealing chamber and annealed at 480°C. for a period of three minutes. The chamber was flooded with argon at3 Torr during annealing to prevent the exposed titanium layer fromreacting with other materials.

The reflectance curve of the exposed annealed metal stack, as measuredwith a spectral analyzer (Model UV1050, Tencor, Boise, Id.) according tothe procedures discussed above, is depicted in FIG. 4. Line 40 showsthat the reflectance of the stack is at or below 10% over the 100-450 nmwavelength range.

EXAMPLE 2

In another method according to the present invention, a 300 Å thicklayer of titanium was deposited on a generally planarboronphosphosilicate glass substrate using direct current magnetronsputtering. A 5000 Å thick layer of aluminum was deposited on thetitanium layer using direct current magnetron sputtering in the sametool, after which a second layer of titanium having a thickness of about500 Å was then deposited over the aluminum layer using direct currentmagnetron sputtering in the same chamber used to deposit the firsttitanium layer. All layers were substantially homogenous in composition.

The wafer was then placed in an annealing chamber and annealed at 480°C. for a period of three minutes. The chamber was flooded with argon at3 Torr during annealing to prevent the exposed titanium layer fromreacting with other materials.

The reflectance curve of the exposed annealed meta stack, es measuredaccording to the procedures discussed above, is depicted in FIG. 5. Line50 shows that the reflectance of the stack is at or below about 12% overthe 100-450 nm wavelength range.

EXAMPLE 3

In another method according to the present invention, a 500 Å thicklayer of titanium was deposited on a generally planarboronphosphosilicate glass substrate using direct current magnetronsputtering. A 5000 Å thick layer of aluminum was deposited on thetitanium layer using direct current magnetron sputtering in the sametool, after which a second layer of titanium having a thickness of about500 Å was then deposited over the aluminum layer using direct currentmagnetron sputtering in the same chamber used to deposit the firsttitanium layer. All layers were substantially homogenous in composition.

The wafer was then placed in an annealing chamber sand annealed at 480°C. for a period of two minutes. The chamber was flooded with nitrogen at3 Torr during annealing to promote the formation of Ti_(x) Al_(1-x)N_(y) and Ti_(x) N_(1-x) in the metal stack.

The reflectance curve of the exposed annealed metal stack, as measuredaccording to the procedures discussed above, is depicted in FIG. 6. Line60 shows that the reflectance of the stack is at or below about 2% overthe 100-450 nm wavelength range.

EXAMPLE 4

In another method according to the present invention, a 500 Å thicklayer of titanium was deposited on a generally planarboronphosphosilicate glass substrate using direct current magnetronsputtering. A 5000 Å thick layer of aluminum was deposited on thetitanium layer using direct current magnetron sputtering in the sametool, after which a second layer of titanium having a thickness of about500 Å was then deposited over the aluminum layer using direct currentmagnetron sputtering in the same chamber used to deposit the firsttitanium layer. All layers were substantially homogenous in composition.

The wafer was then placed in an annealing chamber and annealed at 480°C. for a period of three minutes. The chamber was flooded with nitrogenat 3 Torr during annealing to promote the formation of Ti_(x) Al_(1-x)N_(y) and Ti_(x) N_(1-x) in the metal stack.

The reflectance curve of the exposed annealed metal stack, as measuredaccording to the procedures discussed above, is depicted in FIG. 7. Line70 shows that the reflectance of the stack is at or below about 2% overthe 100-450 nm wavelength range.

COMPARATIVE EXAMPLE A

A layer of TiN having a thickness of about 250 Å was sputtered over alayer of aluminum having a thickness of about 5000 Å to provide ananti-reflective coating.

The reflectance of the exposed TiN layer bear measured according to theprocedures discussed above and the results depicted in FIG. 8. Line 80shows that the reflectance of the TiN layer to reach a minimum of about9% at a wavelength of about 350 nm. Reflectance rises rapidly, however,on either side of the minimum as compared to the relatively lowreflectance obtained over the range of about 100 to about 450 nm byannealing titanium and aluminum according to the present invention.

Although specific methods and examples have been illustrated anddescribed herein, it will be appreciated by those of ordinary skill inthe art that any arrangement that is calculated to achieve the samepurpose may be substituted for the specific methods and examplesdescribed. This application is intended to cover any adaptations orvariations of the present invention. Therefore, it is manifestlyintended that this invention be limited only by the claims and theequivalents thereof.

What is claimed is:
 1. A method of reducing the reflectance from asemiconductor wafer comprising the steps of:a) providing a layer ofaluminum on a semiconductor wafer; b) providing a layer of titaniumabove the aluminum layer; and c) annealing the aluminum and titaniumlayers, wherein the reflectance of the exposed annealed layers is lessthan about 10% over wavelengths of about 100 to about 450 nm.
 2. Amethod according to claim 1, wherein the reflectance of the exposedannealed layers is less than about 5% over wavelengths from about 100 toabout 450 nm.
 3. A method according to claim 1, wherein the annealing isperformed in a non-reactive atmosphere.
 4. A method according to claim1, wherein the annealing is performed in a nitrogen atmosphere.
 5. Amethod according to claim 1, wherein the titanium layer is depositeddirectly on the aluminum layer.
 6. A method according to claim 1,further comprising the step of providing a base layer of titaniumbetween the aluminum layer and the semiconductor water.
 7. A methodaccording to claim 1, wherein the annealing is performed at temperaturesof about 350 to about 550° C.
 8. A method according to claim 1, whereinthe annealing is performed at temperatures of about 440 to about 480° C.9. A method according to claim 1, wherein the step of annealing formscompounds comprising both titanium and aluminum.
 10. A method accordingto claim 1, wherein the step of annealing forms compounds comprisingtitanium, aluminum, and nitrogen.
 11. A method of reducing thereflectance from a semiconductor wafer comprising the steps of:a)providing a first layer of titanium on a semiconductor wafer; b)providing a layer of aluminum directly on the first titanium layer; c)providing a second layer of titanium directly on the aluminum layer;and; d) annealing the aluminum and titanium layers at temperatures ofabout 350 to about 550° C., wherein the reflectance of the exposedannealed layers is less than about 10% over wavelengths of about 100 toabout 450 nm.
 12. A method according to claim 11, wherein the thicknessof the second titanium layer is about 100 to about 1000 Å.
 13. A methodaccording to claim 11, wherein the thickness of the second titaniumlayer is about 200 to about 750 Å.
 14. A method according to claim 11,wherein tile step of annealing forms compounds comprising both titaniumand aluminum.
 15. A method according to claim 11, wherein the step ofannealing forms compounds comprising titanium, aluminum, and nitrogen.16. A method of reducing the reflectance from a semiconductor wafercomprising the steps of:a) providing a layer of aluminum on asemiconductor wafer; b) providing a layer of titanium directly on thealuminum layer; and c) annealing the aluminum and titanium layers in anitrogen atmosphere, wherein the reflectance of the exposed annealedlayers is less than about 10% over wavelengths of about 100 to about 450nm.
 17. A method of reducing the reflectance from a semiconductor wafercomprising the steps of:a) providing a layer of aluminum on asemiconductor wafer; b) providing a layer of titanium directly on thealuminum layer; and c) annealing the aluminum and titanium layers toform compounds comprising both titanium and aluminum, wherein thereflectance of the exposed annealed layers is less than about 10% overwavelengths of about 100 to about 450 nm.
 18. A method of reducing thereflectance from a semiconductor wafer comprising the steps of:a)providing a layer of aluminum on a semiconductor wafer; b) providing alayer of titanium directly on the aluminum layer; and c) annealing thealuminum and titanium layers in a nitrogen atmosphere to form compoundscomprising titanium, aluminum, and nitrogen, wherein the reflectance ofthe exposed annealed layers is less than about 10% over wavelengths ofabout 100 to about 450 nm.